#maindiv {
width: 760px;
height: 760px;
float: left;
}

.card {
width: 180px;
height: 180px;
margin-right: 4px;
margin-bottom: 4px;
float: inherit;
background-image: url('memorycard.png'); 
}

.card:hover {
width: 180px;
height: 180px;
margin-right: 4px;
margin-bottom: 4px;
float: inherit;
background-image: url('memorycardhover.png'); 
}


